There are two good reasons for not using the combination "pretend as if"--one lexical and the other grammatical. These reasons, however, don't deter a fair number of native speakers from using it on occasion.
1) In purely lexical terms, "pretend" doesn't work with "as if" because it would be redundant. "Pretend," means "to give an appearance of (something that is not true), with the intention of deceiving."* Similarly, "[act/look/behave] as if" introduces an idea that is not seen at the moment as valid (whether or not it is later proven valid).
2) "Pretend" has three types of complement: pretend to + verb (he pretended to enjoy the meal); pretend that + clause (she pretended that she didn't know he cared); and pretend + noun (The witness pretended ignorance of the defendant's financial problems)
This fact doesn't seem to bother a great many users of English. Google yields 742 examples of "pretended as if"; 509 of "pretends as if"; and 4,540 examples of "pretend as if." Apparently many native speakers of English are comfortable with the redundancy.
Nevertheless, I would not recommend teaching this combination for active production, since it is an example of less-than-precise usage.
Sentences 4 and 5, which use "look to be" and "look as if," respectively, are different from the other three in that they are not about the creation of a false impression. They express the speaker/writer's belief that something may very well be the case.
*Longman Dictionary of Contemporary English, (1987)